1. Field of the Invention
This invention relates to a covered electrode which is particularly suitable for use in arc welding of 2.25-3% Cr-1% Mo high strength steels to form a weld metal with excellent properties in high temperature strength and toughness.
2. Prior Art
Because of the excellent high temperature properties, 2.25-3% Cr-1% Mo steel has been applied as a material for boilers, pressure vessels, and chemical reactors.
Such 2.25-3% Cr-1% Mo steels, however, are unsatisfactory for application to pressure vessels which are used under higher temperature and pressure conditions, for example, as in a coal liquefaction plant. In an attempt to meet the requirements in this regard, there have been developed 2.25-3% Cr-1% Mo high strength steels which are imparted with higher strength by addition of V and Nb and at the same time improved in properties against hydrogen attacks.
On the other hand, there have been proposed a number of welding materials for the steels of the sort mentioned above. For example, Japanese Laid-Open Patent Applications 62-137196 and 62-161496 propose a covered electrode for use in arc welding of Cr-Mo type low alloy steels.
The improvement of the high temperature properties like creep rupture strength of the welding material simply by addition of V and Nb, however, invites an excessive increase in room temperature strength, giving rise to a problem of increased sensitivity to hydrogen embrittlement.